The effects of hydrogen diffusion into optical fibers have been studied for more than a decade. Four principal aspects have been the objects of these studies: enhancing the photosensitivity of optical fibers, Bragg wavelength shift due to an increase of the effective index of optical fiber, absorption losses increase and hydrogen sensing.
More specifically, the presence of hydrogen molecules in the optical fiber changes its effective refractive index (nef). When a Bragg grating is present in the optical fiber, the resulting reflected Bragg wavelength (λB) as described by the Bragg condition equation is affected by the refractive index change:λB=2neffΛ  (1)where Λ is the period between the fringes of different refractive indexes.
Another effect of the presence of molecular hydrogen into an optical fiber is the increased attenuation of the light energy traveling through it, via molecular absorption. This phenomenon has an impact on fiber optics based monitoring systems for oil and gas extraction where pressure, temperature and environmental hydrogen concentration are usually quite high.
It is known to use either Bragg wavelength shift or absorption losses increase due to the presence of molecular hydrogen in the core of an optical fiber to monitor the concentration of hydrogen in the environment, or in other substances that would chemically react to create hydrogen. This type of sensors can have a very slow dynamic reactive time because it is limited by the diffusion rate of hydrogen through the glass of the optical fiber until it reaches the core of the fiber. Typically it takes a month to reach 97.3% of the equilibrium concentration of hydrogen for a standard 125 microns diameter fiber at 20° C. For that reason, most hydrogen sensors based on fiber Bragg gratings use a hydrogen-reactive coating, such as a palladium-based coating, that swells by absorbing hydrogen, and so, strain the fiber segment containing the Bragg grating increasing the refractive indexes fringes (Λ).
There is a need in the industry to provide optical measuring devices that can operate in sensing zones containing hydrogen, that are of simple construction, are reliable and provide accurate measurements results.